1. Field of the Invention
The present invention generally concerns magnetic resonance tomography (MRT) as used in medicine for the examination of patients. The present invention is in particular concerned with an acquisition technique in which the conventional MR (angiography) acquisition technique is improved with continuous table displacement.
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear magnetic resonance and has been successfully used as an imaging method in medicine and biophysics for over 15 years. In this examination method the subject is exposed to a strong, constant magnetic field. The nuclear spins of the atoms in the subject, which previously were randomly oriented, thereby align. Radio-frequency energy can now excite these “ordered” nuclear spins to a specific oscillation. This oscillation generates the actual measurement signal in MRT which is received by means of suitable acquisition coils. The measurement sub can be spatially coded in all three spatial directions by the use of spatially linearly variable magnetic fields generated by gradient coils, which is generally designated as “spatial coding”.
The acquisition of the data in MRT ensues in k-space (frequency space). The MRT image in image space is linked with the MRT data in k-space by Fourier transformation. The spatial coding of the subject which spans k-space ensues by means of gradients in all three spatial directions. The slice selection (establish an acquisition slice in the subject, for example the z-axis), the frequency coding (establish a direction in the slice, for example the x-axis) and the phase coding (determine the second dimension within the slice, for example the y-axis) are thereby differentiated. Moreover, the selected slice can be subdivided into partitions in 3D imaging via additional phase coding, for example along the z-axis.
A slice (for example in the z-direction) is thus initially excited, and a phase coding in, for example, the x-direction is possibly implemented. The coding of the spatial information in the slice ensues via a combined phase coding and frequency coding by means of these two aforementioned orthogonal gradient fields that, in the example of a slice excited in the z-direction, are generated in the x-direction and y-direction by the (likewise aforementioned) gradient coils.
In order to measure an entire slice of the subject to be examined, the imaging sequence (for example gradient echo sequence, FLASH) is repeated N times for different values of the phase coding gradients (for example Gy). The time interval of the respective excited RF pulses is thereby designated as a repetition time TR. The magnetic resonance signal (for example gradient echo signal) is likewise scanned in the presence of the readout gradient GF, digitized and stored N times at equidistant time steps Δt in each sequence pass. In this way a number matrix (matrix in k-space or k-matrix) generated line for line with N×N data points is obtained. From this data set an MR image of the considered slice with a resolution of N×N pixels can be reconstructed from this data set via a Fourier transformation (a symmetrical matrix with N×N points is only one example; asymmetrical matrices can also be generated). For physical reasons the values in the region of the center of the k-matrix primarily contain information about the contrast; the values in the border region of the k-matrix predominantly contain information with regard to the resolution of the transformed MRT image.
Slice images of the human body can be acquired in all directions in the manner just described above. MRT as a slice image method in medical diagnostics is primarily characterized as a “non-invasive” examination method. Nevertheless, particularly in angiographic acquisitions (i.e. acquisitions of the blood vessels in the human body, especially in perfused organs) limits are set on the contrast level in non-enhanced MR imaging. The contrast level, however, can be significantly enhanced by the use of contrast agent. The use of contrast agents in magnetic resonance tomography is generally based on effecting the parameters that are significant to the contrast, for example the longitudinal and transversal relaxation times T1 and T2. In clinical application, trivalent gadolinium Gd3+—that has a T1-shortening effect—has become established. By integration into chelate complexes (DTPA, diethylentriaminepentaacetic acid), gadolinium loses its toxicity, such that Gd-DTPA can normally be administered intravenously. A vein is chosen that leads directly to the heart, which ultimately distributes the contrast agent in the entire arterial system (normally from the aortic arch to the tips of the toes). In prevalent sequences (T1-weighted spin echo sequence, gradient echo sequence etc.) the accelerated T1 relaxation produces an increase of the MR signal, thus a lighter depiction of the appertaining tissue in the MR image. In this way sharp and high-contrast images can be measured, for example of the head, neck, heart or kidney vessels.
Such a contrast agent-assisted method in magnetic resonance tomography is generally designated as “contrast-enhanced angiography” (Contrast Enhanced MR Angiography, CE MRA). The quality of contrast agent-assisted vessel exposures essentially depends on the temporal coordination of the sequence steps characterizing the measurement, which is generally designated as timing or contrast agent timing. The decisive sequence steps are: contrast agent injection; measurement duration and measurement of the center of the k-space matrix. In order to achieve an optimally good contrast of the acquisition, it is sought that a maximum contrast agent concentration in the region of interest that is to be acquired (FOV, field of view) is present during the measurement of the middle region of the k-matrix. For this reason a contrast-enhanced angiography according to the prior art is implemented as follows.
A contrast agent is intravenously injected into the body, and the contrast agent distributes uniformly through the arterial vessel system via the heart (in particular from the aortic arch to the ends of the feet). It is sought to track the contrast agent enrichment (also designated as a “bolus”) by means of an MR measurement by blocks of the body region being successively excited in the FOV of interest. After the measurement of a block, the patient is shifted by, for example, the block width in the head direction by table displacement, and a new vessel segment in the form of a next block of the same direction is excited and measured. The measurement of a 3D block with, for example, a width of 10 to 15 cm given a FOV of 400 to 500 mm leads to an acquisition time from 10 to 20 s per station, such that the measurement of the entire body from the heart to the ends of the feed amounts to approximately 1 to 1.5 minutes.
This step-by-step, multi-station, whole-body imaging has certain disadvantages: valuable time that is actually necessary for data acquisition is lost due to the time that the table displacement requires. Valuable measurement time is likewise lost because a steady-state signal must first be established at every station, and the FOVs of a station (table position) overlap must with its neighboring stations (in order to ensure a seamless depiction of the anatomy as a whole), which leads to a data acquisition that is redundant in part. A signal decline at the edges of each partial image volume (due to deficiencies of the RF coils) also leads to disruptive signal inhomogeneities in the total FOV. An additional disadvantage of this technique is that gradient nonlinearities lead to geometric distortions at the edges of the respective partial volumes and result in border artifacts between adjacent blocks.
In order to solve these problems, a method that allows the acquisition of a homogeneous, high-contrast, contiguous large MR image—thus an MR image of an expanded field of view (FOVtot)—is proposed by Krugel et al., wherein the patient table is moved continuously during the scan (Kruger et al.; Journal of Magnetic Resonance in Medicine 2002 February; 47 (2): 224-231). According to this method, all acquired MRT data are corrected for a common table reference position, whereby a single, seamless MRT image can be reconstructed. As stated, a single MRT image can be generated in this way over a spatial region which far exceeds the normal FOV of the MRT apparatus. The patient is moved continuously through the MRT apparatus, and both table and views are simultaneously acquired. Every view is corrected according to position using the associated table position data in order to generate an individual table matrix of MRT data, which is used for image reconstruction. This technique is designated by some manufacturers as “TIM Technology” (Total Imaging Matrix, TIM) or “TIMCT” (TIM with Continuous Table Movement).
However, the aforementioned method is limited because the scanning in the readout direction (frequency coding direction) has to ensue exclusively in the table movement direction. The method according to Kruger et al. has the disadvantage that only 2D slices or 3D volumes with coronal or sagittal orientation can be excited and measured by MR as a planning FOV (FOVtot or presentation region or target volume) and as a partial FOV (also called “RF excitation volume, RF-EV” for the respective phase coding step in the following).
A rectangular planning FOV in a sagittal slice of a patient is shown in FIG. 2. FIG. 3 shows a number of likewise rectangular RF excitation volumes (RF-EVs) strung along in series or partially overlapping via which the planning FOV is optimally and completely covered (overlapped). From FIG. 3 it is clear that a measurement of the planning FOV (FOVtot) ensues with multiple technically realizable, smaller RF excitation volumes. An overlapping of these small RF excitation units among one another and with the space outside the planning FOV is possible. Finally, the Fourier-transformed segments of the respective phase coding steps that have occurred in the table displacement direction (here the x-direction) are shown in FIG. 4, sorted in the x-ky hybrid space at the respective, actual pixel position. Each of these segments HA represents one RF excitation that corresponds to a phase coding step, wherein an RF excitation volume according to FIG. 3 is associated with each of these phase coding steps; HA1 corresponds to RF-EV1, HA2 corresponds to RF-EV2 etc. An additional phase coding in the z-direction ensues given a 3D overlapping.
In contrast agent-based MRT methods (CE-MRA methods; Contrast Enhanced Magnetic Resonance Angiography) large horizontal segments sometimes must be measured (for example from the head or heart to the extremities, hands or feet). These are thus segments in which the vessels to be imaged (for example a vessel tree consisting of veins) can also have a sagittal and/or coronal course component (for example a vertical and/or horizontal course component as viewed from a sagittal or coronal viewing direction) in a direction orthogonal to the longitudinal patient axis (depending on the view, for example radially in the sagittal plane or in the coronal plane), as well as an axial course in the longitudinal patient direction. In such situations, the actual regions to be measured must be selected extremely large in the method according to Kruger et al. For the geometric reasons described above, the planning FOV according to Kruger must always be able to be registered by a rectangle parallel to the longitudinal axis of the patient or, respectively, by a corresponding parallel cuboid.
This in turn means that far more (in the worst case a multiple of) measurement data must be acquired than would actually be necessary for a mere vessel depiction. For example, in FIG. 5 a vessel tree stretching from the torso to the feet should be acquired by means of MRT angiography. A planning FOV (FOVtot2) circumscribes the vessel tree to be acquired by means of CE-MRT measurement; although this vessel tree is only slightly angled, the method according to Kruger et al. requires more than twice as large a presentation area (target volume FOVtotKruger2) in the sagittal slice, which can make the method according to Kruger et al. extremely inefficient. The disadvantages are longer measurement times and a high data level that must be additionally stored and evaluated. Even a planning FOV (FOVtot3) that is better adapted to geometric course of the vessel tree (as is shown in FIG. 7) does not significantly reduce the target volume that is still required according to Kruger (FOVtotKruger3) in comparison to the target volume FOVtotKruger2 (FIG. 6), as is apparent from FIG. 8.
Compared to a multi-station MRT method, the method according to Kruger thus has the significant disadvantage that the MRT measurement of the respective anatomy of the patient cannot be optimally adapted, as is possible in MRT imaging with individual stations.
In order to keep this disadvantage minimal, conventionally the sagittal vertical extent or the coronal vertical extent of the total FOVs inscribing the planning FOV is reduced or minimized. This is implemented, for example, by the patient being supported substantially flat and level by means of suitable supports and cushions, which requires a laborious, and imprecise, procedure. In the case of CE angio, and therefore coronal imaging, according to the conventional technique the patient must thus be carefully supported in order to minimize the FOV in the anterior-posterior dimension.